The United States renewed a travel warning on Tuesday for U.S. citizens crossing the border into Mexico, noting more than 100 murders had occurred in the border region since June.

The U.S. ambassador in Mexico, Tony Garza, said that with violence showing no sign of abating, the travel warning issued twice by Washington since the start of the year had been extended and would be reviewed as necessary.

"The Mexican government is beginning to address concerns about the unacceptable level of violence along our border. Nevertheless, more than 100 violent deaths have occurred along the border since June," Garza said.

"Eighteen Nuevo Laredo policemen were killed this year, including eight in July. These disturbing reports make clear that Mexico needs to do much more to bring safety and security to our common border."

Mexico, which has reacted angrily to previous travel alerts and accused Washington of interfering in its business, retorted with a short statement saying it was continuing its efforts to fight organized crime.

"As it has reiterated on various occasions, the government maintains a constant fight against organized crime and is undertaking a series of actions to quell violence on the border," the foreign ministry said in a short statement.

Violence in northern Mexico has soared this year, leaving at least 600 dead, after President Vicente Fox launched a "mother of all battles" on drug gangs fighting turf battles there.

Fox sent hundreds of anti-drug troops to the area in June after a spate of police shootings in border cities, where cartels from the western state of Sinaloa are battling local Gulf cartel rivals for control of the cross-border cocaine, marijuana and amphetamine trade.

One of the worst trouble spots is Nuevo Laredo, south of Laredo, Texas, where dozens of people have been murdered this year and some 30 U.S. citizens have been kidnapped since last August.